1. Field of the Invention
The present invention relates to a composite material including an organic compound and an inorganic compound, a light-emitting element, a light-emitting device, an electronic device, and a lighting device. The present invention also relates to an organic compound that can be used for the composite material.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using organic electroluminescence (EL). The light-emitting elements have a basic structure in which a layer containing a light-emitting organic compound is interposed between a pair of electrodes. By applying voltage to this element, light can be emitted from the light-emitting organic compound.
Since such a light-emitting element is of self-light-emitting type, it is considered that the light-emitting element has advantages over a liquid crystal display in that visibility of pixels is high, backlight is not required, and so on and is therefore suitable as a flat panel display element. In addition, it is also a great advantage that the light-emitting element can be manufactured as a thin and lightweight element. Furthermore, very high speed response is also one of the features of the light-emitting element.
Furthermore, since such light-emitting elements can be formed in a film form, they make it possible to easily form a large-area element. This feature is difficult to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements also have great potential as planar light sources applicable to lighting devices and the like.
As described above, application of light-emitting elements using organic EL to light-emitting devices, lighting devices, or the like is expected. At the same time, there are many issues regarding light-emitting elements using organic EL. One of the issues is a reduction in power consumption. In order to reduce power consumption, it is important to reduce driving voltage for the light-emitting element. The emission intensity of the light-emitting element using organic EL is determined by the amount of electric current flowing therein. Therefore, in order to reduce the driving voltage, it is necessary to feed a large amount of current at low voltage.
Previously, as a method of reducing driving voltage, an approach of providing a buffer layer between an electrode and the layer containing a light-emitting organic compound has been attempted. For example, it is known that driving voltage can be reduced by providing a buffer layer which includes polyaniline (PANI) doped with camphorsulfonic acid, between indium tin oxide (ITO) and a light-emitting layer (e.g., see Non-Patent Document 1). It is explained that this is because PANI has an excellent property of injecting carriers into the light-emitting layer. Note that in Non-Patent Document 1, PANI, which is used for the buffer layer, is also regarded as part of the electrode.